The present invention relates to a process for separating or recovering phthalic acid anhydride (PA) contained in the form of vapor in a gas stream by cooling the gas stream and solidifying the PA in a cooler which contains a fluidized bed that consists of granules containing PA, where the fluidized bed is cooled indirectly.
Such a process is known from GB-A-988084. In accordance with this process, the gas stream containing PA is introduced into the lower portion of a cooled fluidized bed. However, the gas is forced through the fluidized bed largely in the form of bubbles which exhibit a very stable behavior. The gas bubbles prevent an intensive heat transfer between the PA vapor contained in the bubbles and the relatively cold solid particles of the fluidized bed. The cooling of the PA vapor is therefore insufficient, and experience has shown that one can only achieve a condensation of nor more than 50% of the PA vapor in the fluidized bed.
It is the object underlying the invention to intensively cool the gas stream containing vaporous PA by means of an indirectly cooled fluidized bed, so that the PA is separated from the gas stream with a high efficiency. In accordance with the invention this is achieved in that the gas stream containing vaporous PA is passed in an upward direction through a vertical tube disposed in the lower portion of the cooler, that the vertical tube has an upper orifice, that the tube and its orifice are surrounded by the indirectly cooled fluidized bed, whose temperature lies in the range from 20 to 90xc2x0 C. and to which fluidizing gas is supplied from the bottom, where the suspension density of the fluidized bed lies in the range from 300 to 700 kg/m3, that the inner portion of the tube has no fluidized bed, that from the fluidized bed through the orifice of the vertical tube granules of the fluidized bed are constantly introduced into the gas stream containing vaporous PA and are supplied by the gas stream to a settling space disposed in the cooler upstream of the tube and above the fluidized bed, where vaporous PA contained in the gas stream is cooled and solidified, and where solidified PA at least partly drops from the settling space onto the fluidized bed, that gas is discharged from the settling space and from the cooler, and that granules containing PA are withdrawn from the fluidized bed.
By means of the inventive process, more than 90% of the introduced PA vapor is cooled and solidified in the cooler. Usually, 10 to 50 kg solids per Nm3 gas will be introduced from the fluidized bed in the vicinity of the orifice of the vertical tube into the gas stream containing PA. The settling space and also the area directly above the orifice of the vertical tube is free from a fluidized bed. There are only relatively small amounts of solids, so that gas bubbles cannot be formed there as they are, however, inevitable in a fluidized bed.
Advantageously, the granules of the fluidized bed comprise at least 80 wt-% grain sizes of not more than 1 mm, when no auxiliary granules are employed. The relatively fine-grained granules have a good flowability and can be indirectly cooled in the fluidized bed with high heat transfer coefficients.
For fluidizing the fluidized bed various gases may be used. Expediently, there is used at least partly deducted gas withdrawn from the cooler, or air, or a mixture of these two gases.
The gas stream containing the vaporous PA usually comes from a reactor for catalytically producing PA from orthoxylene or naphthalene with air. The gas stream containing vaporous PA, which is produced in this known way, can first of all be cooled indirectly in one or several stages in a waste heat boiler before it is introduced into the vertical tube for final cooling. A precooling without condensation and without producing solid PA may be advantageous when it is desired to reduce the thermal load in the final cooling.
A cooler of the type used in the inventive process for solidifying the PA vapor in described in EP-B-0467441. This cooler is in particular provided for cooling an exhaust gas from the process of smelting lead ore, where the purification of gas chiefly meets the requirements of environmental protection. It has now been discovered that the basically known cooler is able to solidify relatively large amounts of PA, which are supplied in the form of vapor.
For producing the fluidized bed in the cooler, which surrounds the vertical tube, one can omit auxiliary granules or employ such auxiliary granules, e.g. sand with grain sizes of about 0.05 to 1 mm. In the cooled fluidized bed, PA in condensed on the auxiliary granules and is withdrawn together with the same. Outside the fluidized bed the crude PA is separated from the auxiliary granules, e.g. by melting it off, and the auxiliary granules can be recirculated to the fluidized bed. When no auxiliary granules are employed, this separation step is omitted.